On August 12th 2021 a new American military assessment said:
“Kabul could come under Taliban pressure as soon as September and, if current trends hold, the country could fall to the Taliban within a few months.”
Within days of that assessment Kabul had fallen to the Taliban. Why was it that Kabul fell so soon: how could the Taliban take over so quickly? The short answer is that it seems that the Afghan government had concluded that fighting was pointless.
But the main and more fundamental reason for the speed of the victory is to do with the fact that the Taliban, although hated and feared by large numbers of people in Afghanistan, are actually popular. This is because, I suggest, the vast majority of the population of Afghanistan wants to be ruled with Islamic Sharia law.
For those people in Afghanistan who want to want to live a secular or Christian lifestyle, Afghanistan is not safe. And many of them have already left to live in other countries. Those who are still there may find their freedoms curtailed since, according to Pew research, 99% of the population of Afghanistan want Islamic Sharia law to be enforced in their country.
This meant that, when the American troops were leaving, the Taliban rapidly took over the entire country, not because of military superiority Taliban but because there is not much resistance within the country. This was exacerbated by the US administration under Joe Biden reneging on a deal negotiated with the Afghan government and the Taliban on the timing of withdrawal. This breaking of the agreement made the American backed Afghanistan government seem illegitimate in the eyes of many there, it seem. And this in turn gave good material for a Taliban recruitment drive.
In the face of this popular revolt, the Afghan leaders left. Now the people they once led are under the control of that tiny minority of extremists who fought the Soviet Union for nine years. This group, now called the Taliban, then fought the United States of America for two decades and won. They didn’t win in the sense that they defeated the two most powerful nations in history on the battlefield. They won by wearing down the will of the countries they fought against. They won by being patient and relentless. And they won this time because the majority of Muslims in the country prefer Sharia Law to an American backed government.
I based these remarks on the insights from people more knowledgeable about Islam, especially see this video.